The overall catalytic capacity of the rate-limiting step in a metabolic pathway is the product of the concentration of enzyme molecules and their intrinsic catalytic efficiency. It therefore follows that catalytic capacity can be controlled by changing the quantity of enzyme present, altering its intrinsic catalytic efficiency, or a combination thereof.
Proteins Are Continuously Synthesized & Degraded
By measuring the rates of incorporation and subsequent loss of 15N-labeled amino acids into proteins, Schoenheimer deduced that proteins exist in a state of “dynamic equilibrium” where they are continuously synthesized and degraded—a process referred to as protein turnover. Even constitutive proteins, those whose aggregate concentrations remain essentially constant over time, are subject to continual replacement through turnover. However, the concentrations of many other enzymes are subject to dynamic shifts in response to hormonal, dietary, pathologic, and other factors that may affect the overall rate constants for their synthesis (ks ), degradation (kdeg ), or both.
Control of Enzyme Synthesis
The synthesis of certain enzymes depends on the presence of inducers, typically substrates or structurally related com pounds that stimulate the transcription of the gene that encodes them, or transcription factors. Escherichia coli grown on glucose will, for example, only catabolize lactose after addition of a β-galactoside, an inducer that triggers synthesis of a β-galactosidase and a galactoside permease. Inducible enzymes of humans include tryptophan pyrrolase, threonine dehydratase, tyrosine-α-ketoglutarate aminotrans ferase, enzymes of the urea cycle, HMG-CoA reductase, Δ-aminolevulinate synthase, and cytochrome P450. Conversely, an excess of a metabolite may curtail synthesis of its cognate enzyme via repression. Both induction and repression involve cis elements, specific DNA sequences located upstream of regulated genes, that provide binding sites for trans-acting regulatory proteins. The molecular mechanisms of induction and repression are discussed in Chapter 38, while detailed information on the control of protein synthesis in response to hormonal stimuli can be found in Chapter 42.
Control of Enzyme Degradation
In animals many proteins are degraded by the ubiquitin proteasome pathway. Degradation takes place in the 26S proteasome, a large macromolecular complex made up of more than 30 polypeptide subunits arranged in the form of a hollow cylinder. The active sites of its proteolytic subunits face the interior of the cylinder, thus preventing indiscriminate degradation of cellular proteins. Proteins are targeted to the interior of the proteasome by the covalent attachment of one or more molecules of ubiquitin, a small, 76 amino acid (≈ 8.5-kDa), protein that is highly conserved among eukaryotes. “Ubiquitination” is catalyzed by a large family of enzymes called E3 ligases, which attach ubiquitin to the side-chain amino group of lysyl residues on their targets.
The ubiquitin-proteasome pathway is responsible both for the regulated degradation of selected cellular proteins, for example, cyclins, and for the removal of defective or aberrant protein species. The key to the versatility and selectivity of the ubiquitin-proteasome system resides in the variety of intracellular E3 ligases and their ability to dis criminate between the different physical or conformational states of target proteins. Thus, the ubiquitin-proteasome path way can selectively degrade proteins whose physical integrity and functional competency have been compromised by the loss of or damage to a prosthetic group, oxidation of cysteine or histidine residues, partial unfolding, or deamidation of asparagine or glutamine residues. Recognition by proteolytic enzymes also can be regulated by covalent modifications such as phosphorylation; binding of substrates or allosteric effectors; or association with membranes, oligonucleotides, or other proteins. Dysfunctions of the ubiquitin-proteasome path way sometimes contribute to the accumulation and subsequent aggregation of misfolded proteins characteristic of several neu rodegenerative diseases.
